Lecture 10 - Rankine, Brayton, CC, and CHP PDF

Rankine, Brayton, Combine Cycles, and CHP ME 4180 – Energy Systems and Sustainability Prof. Mario Medina Department of Mechanical Engineering Sept. 30, 2020 Agenda Objective Review the basics of heat and power cycles for stationary and transportati...

Rankine, Brayton, Combine Cycles, and CHP ME 4180 – Energy Systems and Sustainability Prof. Mario Medina Department of Mechanical Engineering Sept. 30, 2020 Agenda Objective Review the basics of heat and power cycles for stationary and transportation sectors Agenda Steam power plants: Rankine cycle Gas turbine power plants: Brayton cycle Combine cycle power plants Combine heat and power: cogeneration Steam power plants The Rankine cycle: the working fluid is steam or water Analyze boiler or steam heater Critical step: heat from energy carrier to working fluid Gas turbine power plants The basic Brayton cycle: the working fluid is air Cycle efficiency increases with increasing burner operating pressure. Compressor work is a much higher fraction of the turbine work compared to pump work in a Rankine cycle (back work ratio) Gas turbine power plants Improving the basic Brayton cycle Wcycle Wturb ,1 + Wturb ,2 + Wcomp ,1 + Wcomp ,1 ηcycle =  = Q in Q + Q cc ,1 cc ,2 Multi-stage compression with intercooling Multi-stage expansion with reheat Regenerator (counterflow heat exchanger) In the limit of infinite intercooling/reheat, the Brayton cycle approaches isothermal heat addition and isothermal heat removal. Combined cycle power plants Our first effort at waste heat recovery  Waste heat recovered through combined cycles More than 20% of the U.S. total energy use in 1997 was wasted in thermal losses from power plants. That is enough energy to power almost all of the transportation sector.* 60% Waste heat Fuel Coal-fired 40% Electricity out ηRankine = 40% 100% Steam Power Plant Combined Cycle Fuel Natural Gas Turbine 30% Electricity out 100% (Brayton) 70% Waste heat ηCC = 55-60% Steam Power Plant 70×40=28% Electricity out (Rankine) *Shepard and Shepard, 2003 70×60=42% Waste heat Combined cycle power plants Video tour of a combine cycle power plant https://www.youtube.com/watch?v=KVjtFX We9Eo Combined heat and power (CHP) Generating power always creates heat Combined heat and power plants are also called co-generation plants In CHP s the heat transfer is a desired product of the cycle CHP can be used at any scale – industrial power plants to I.C. engines and fuel cells But the heat transfer application needs to be co-located with the power source. Heat cannot be transferred large distances without incurring high losses Basic coal fired Combined cycle Combined heat power plant power plant and power plant Combined heat and power (CHP) Comparing generating heat and power independently to combined heat and power (CHP) 144 kWH vs. 100 kWh to generate the same useful energy and heat: 31% energy savings! CHP implemented with NG or steam turbines CHP = Cogeneration – Why not use waste Gas Turbine with Heat Recovery heat for heating? Heat exchangers have high efficiencies (80- 90%) On-site generation of electricity Waste-heat recovery for heating, cooling, air- conditioning, process applications Steam Power Plant with Heat Recovery Advantage: Power and heat are generated from one fuel source. Limitation: This is a distributed power generation model with co-located demand for power and heat (heat cannot go too far) Image source: http://www.epa.gov/chp/basic/index.html CHP/Cogeneration Examples Image source: http://www.utilities.cornell.edu/utl_cchp_how_graphic.html